Page 223 - Mechanical Engineers' Handbook (Volume 4)

212 Furnaces

1 SCOPE AND INTENT
This chapter has been prepared for the use of engineers with access to an electronic calculator
and to standard engineering reference books, but not necessarily to a computer terminal. The
intent is to provide information needed for the solution of furnace engineering problems in
areas of design, performance analysis, construction and operating cost estimates, and im-
provement programs.
In selecting charts and formulas for problem solutions, some allowance has been made
for probable error, where errors in calculations will be minor compared with errors in the
assumptions on which calculations are based. Conscientious engineers are inclined to carry
calculations to a far greater degree of accuracy than can be justified by probable errors in
data assumed. Approximations have accordingly been allowed to save time and effort without
adding to probable margins for error. The symbols and abbreviations used in this chapter
are given in Table 1.

2 STANDARD CONDITIONS
Assuming that the user will be using English rather than metric units, calculations have been
based on pounds, feet, Btus, and degrees Fahrenheit, with conversion to metric units provided
in the following text (see Table 2).
Assumed standard conditions include ambient temperature for initial temperature of
loads for heat losses from furnace walls or open cooling of furnace loads—70 F. Condition
of air entering system for combustion or convection cooling: temperature, 70 F; absolute
pressure, 14.7 psia; relative humidity, 60% at 70 F, for a water vapor content of about 1.4%
by volume.

2.1 Probable Errors
Conscientious furnace engineers are inclined to carry calculations to a far greater degree of
accuracy than can be justified by uncertainties in basic assumptions such as thermal prop-
erties of materials, system temperatures and pressures, radiation view factors and convection
coefficients. Calculation procedures recommended in this chapter will, accordingly, include
some approximations, identified in the text, that will result in probable errors much smaller
than those introduced by basic assumptions, where such approximations will expedite prob-
lem solutions.

3 FURNACE TYPES
Furnaces may be grouped into two general types:

1. As a source of energy to be used elsewhere, as in firing steam boilers to supply
process steam, or steam for electric power generation, or for space heating of build-
ings or open space
2. As a source of energy for industrial processes, other than for electric power

The primary concern of this chapter is the design, operation, and economics of industrial
furnaces, which may be classified in several ways: